Have you noticed your butt is getting flatter, or disappearing altogether? And not in a good way, as in exercise-induced fat loss?

Sure, you can blame age - gravity does cause bits of the body to sag eventually - but there can be another, avoidable, factor at play.

It's having weak gluteal muscles. Even though the glutes are a primary movement muscle that have power and mass and strength, it's easy to neglect them. Unlike other more obvious muscle groups, they are hidden from view by the bottom cheeks and are difficult to work on in isolation. Plus, it's easy to get into the habit of recruiting other muscles such as the back, quads and hip flexors to do the glutes' work.

Take this test: can you clench your butt cheeks? And can you clench each one independently?

If you can't, you're not alone.

Improve the glute

Sydney-based physiotherapist, ironman triathlete and athletics coach Ken Raupach says most people - even runners - wouldn't be able to get a good single individual glute clench.

"It's possible to be a runner with weak glutes by having extremely good quads and strong calves and using your back to gain momentum. And that leads to poor running style," says Raupach. "It's reasonably common."

Raupach says back problems often stem from weak glutes. "The back is a major compensator. You flex and extend through your spine rather than through your hips, and you wear your back out. More than anything else, physios focus on glutes when we are treating people with back problems."

I've certainly had my share of back issues and since working specifically on building up glute strength in the past eight months, I haven't had any. (I've also reduced my running and taken up swimming and cycling again to give the plantar fasciitis a chance to heal). Interestingly, the improved glute strength has helped my running anyway.

Get off your bum

Raupach blames our sitting culture for gluteal neglect.

"Muscles operate best in a neutral position, neither fully extended nor fully flexed. Halfway between the two is where a muscle is most strong. Unfortunately, in the sitting position glutes are not neutral, but fully stretched and being put under pressure: squashed, in fact.

"If you put pressure on a muscle it won't fire up as well when you need it to," says Raupach. "And in a fully stretched position it doesn't have the capacity to generate much power."

The job of glutes is to generate forces - they are the primary hip and leg extension in the walking phase.

"If you do a lot of sitting and not much walking, that glute muscle gets plugged out of the brain's neurological patterns [and] becomes a less favoured muscle," says Raupach. "You get up from a chair and your brain makes a decision to use the most commonly used muscle and it won't be your glutes, it'll be your quads and your back. Hence you develop bad patterns."

How we train our glutes is also important, and the usual methods aren't necessarily the best.

Doing squats and using the leg press at the gym take the glutes from neutral to a fully stretched position."It doesn't extend the leg backwards and that's what we do when we walk and what we are designed to do," says Raupach. "So the exercise we choose to strengthen the glute probably isn't the best position at all."

He says a lunge is better than a squat because at least it gets one leg back, but it is still quad-centric. "It loads up the quads on the front leg," says Raupach.

The best exercise is one that pushes your leg and hip backwards into extension, which is where your glute loves being because that's what it's designed to do - to push backwards when we walk and run. There is a specific apparatus designed for this, though Raupach says it's usually covered in dust against the back wall at the gym, being ignored.

A stretchy Theraband is good, too, which requires resistance to push your leg back behind you. "You're stimulating the muscle more effectively in its anatomically neutral and physiologically correct posture. And it's done standing, which is going to fire it up a little more than if done sitting."

When in doubt, clench

There are incidental ways you can fire up your glutes, too, such as clenching them whenever you stand up from a seated position. And if you sit at work, Raupach suggests doing the following:

"Stand up, put your legs wide apart, squat down to a nice long groin stretch, extend and arch backwards, clench your butts four or five times with a bit of twist each way. I bet anyone who does that will say it feels fantastic."

He says the beauty of that exercise is that it's done in the standing position, and as you extend back, it fires up the extending muscles which are your glutes, your hamstrings and your back. "If you get that posterior chain working, it will maximise the glute firing."

The other interesting and important characteristic of glutes is their stabilising role. "As you get older all your muscles get a bit weaker, but the secret is to have everything in balance," says Raupach. "If you're young and you've got very strong other muscles and weak glutes, that's probably worse than being older and generally weak, but having everything in balance."

Cyclists and rowers often have weak glutes because they are sitting on them and recruiting the back muscles - in a static position in cycling and a dynamic position in rowing. In between training sessions, they are often sitting all day at work or school, where their glutes are again extended and under pressure, and getting no relief.

"Variety in training is the spice of life and glutes are central to everything. It's not just having the strength but being able to switch them on when you need them. They need continual firing up practice," says Raupach.

Plus there's the added incentive of rebuilding your butt into something much more shapely.